As a liquid feeding apparatus for feeding a mobile phase in a liquid chromatograph, there is an apparatus in which two plunger pumps are connected in series. A check valve is provided between a plunger pump on a front stage side (a primary side pump) and a plunger pump on a rear stage side (a secondary side pump). While the primary side pump is performing a liquid discharge operation, the check valve opens, and the liquid from the primary side pump is fed via the secondary side pump. While the primary side pump is performing a liquid discharge operation, the secondary side pump performs a suction operation of the liquid, and a part of the liquid discharged from the primary side pump is sucked by the secondary side pump. Thus, the liquid having a flow rate corresponding to the value obtained by subtracting the suction flow rate of the secondary side pump from the discharge flow rate of the primary side pump is fed from an outlet part of the secondary side pump.
After the discharge operation of the primary side pump is terminated, the suction and discharge operations of the primary side pump and the secondary side pump are switched, and while the secondary side pump is performing the discharge operation, the primary side pump performs the suction operation. At this time, the check valve between the primary side pump and the secondary side pump is closed, and the flow rate of the liquid fed through the outlet part of the secondary side pump becomes the discharge flow rate of the secondary side pump. By this operation, the liquid is continuously fed from the outlet part of the secondary side pump.
When the suction and discharge operations of the primary side pump and the secondary side pump are switched immediately after the suction operation of the liquid using the primary side pump is terminated, the pressure inside a pump chamber of the primary side pump, which has been performing the suction operation just before, becomes lower than the pressure inside a pump chamber of the secondary side pump. Thus, the check valve does not open until the pressure inside the pump chamber of the primary side pump becomes equal to or higher than the pressure inside the pump chamber of the secondary side pump, and a flow fluctuation (pulsating flow) occurs.
Therefore, it has been proposed and practiced that a pressure sensor for detecting the pressure inside the pump chamber of the primary side pump and the pressure inside the pump chamber of the secondary side pump is provided, and after the suction operation of the liquid in the primary side pump is terminated, during the period until the discharge operation in the secondary side pump is terminated, until the pressure inside the pump chamber of the primary side pump becomes equal to the pressure inside the pump chamber of the secondary side pump, a preloading operation of driving the primary side pump to the discharge side is performed (see Patent Document 1).
Patent Document 1: U.S. Pat. No. 5,637,208